(276c) Mixing and Conveying High Solids Biomass Using Rheological Modifiers

Rising global demand for energy combined with an increasing appreciation for the environmental consequences of petroleum-based fuel use is motivating research and development of more carbon-neutral, widely accessible sources of renewable energy. Particularly critical to the transportation industry is an abundant source of renewable liquid fuels to counter rising petroleum costs. A likely feedstock in the production of fuel ethanol is lignocellulosic biomass. Many processes designed to convert lignocellulosic materials to liquid fuel require operations in which biomass slurries are heated (e.g. the NREL process that employs hydrolysis of cellulose). Reducing the amount of water present in those slurries lowers operating costs associated with heating and makes such processes more economical. Processing biomass at high solids concentration leads to a number of difficulties that are not present in more dilute systems. Concentrated biomass acts as a Bingham fluid with a yield stress that increases rapidly with solids concentration. This behavior makes pumping and transporting the process slurry difficult if not impossible. In addition, the mixing required to obtain adequate rates of mass transfer when introducing reactants, enzymes and reagents into the slurry is severely inhibited. Controlling the rheology of biomass will help improve the economics of high solids processing. Prior work on fiber suspensions and recent preliminary work on corn stover slurries have shown that controlling the magnitude of yield stresses present in concentrated biomass slurries is possible using small amounts of high molecular weight polymer additives. We examined the rheological behavior of biomass with polymer additives as a function of process conditions using torque rheometry. Temperature, pH, solids concentration and biomass type (e.g. corn stover vs. switchgrass) were considered as process variables. Water soluble polymers reduced yield stresses in a variety of lignocellulosic materials. Biomass pH and solids concentration significantly affect polymer efficacy. Unusual behavior in the form of a non-monotonic increase of stress with strain rate was observed which can be modified with the addition of polymer. We will also describe more recent experiments in which those rheological modifiers are exploited to facilitate extrusion of biomass at high solids concentrations.